Metallocene olefin polymerization catalysts generally use cyclopentadienyl (Cp), indenyl and fluorenyl groups .pi.-bonded to Group III to VI metals as procatalysts and cocatalysts comprising primarily alumoxanes and boranes. These metallocene catalysts can be used to produce stereospecific polymers from olefins, which is complicated by the structure of these compounds. From the nature of their structure, some compounds of the metallocenes exist as two stereoisomers: a racemic modification and a meso form. The racemic modification includes a pair of enantiomers which is simply a pair of stereoisomers that are nonsuperimposable mirror images. The meso form is a stereoisomer which can be superimposed on its mirror image. Polymerization employing the meso form has traditionally resulted in the production of primarily atactic polymer. See e.g., U.S. Pat. Nos. 5,455,366; 5,329,033; and 5,145,819. See also, Wolfgang A. Herrmann, et al., The First Example of an Ethylene-Selective Soluble Ziegler Catalyst of the Zirconocene Class, 28 Angew. Chem. Int. Ed. Engl., 1511 (No. 11, 1989). Production of polymer primarily of the isotactic form has necessitated use of a catalyst consisting almost exclusively of the racemic modification.
Preparation of metallocene procatalysts has traditionally been conducted in polar solvent systems. See, for example, in European Patent Publication 0 351 392 A. However, due to the aggressive nature of these solvent systems, metallocene procatalysts produced in polar solvents are impure, sensitive to air and moisture, and are susceptible to the decomposition of reaction intermediates, particularly the lithiated ligand, which is generally indicated by a dark colored reaction solution. Additionally, procatalyst yields are also generally low for processes involving a polar solvent system.
Metallocene procatalysts prepared in a polar solvent suffer another difficulty if their intended use is as a catalyst for the production of substantially isotactic polyolefins because the use of these metallocene procatalysts in the polymerization to make stereospecific polymers has typically dictated an additional step in which the racemic modification is separated from the meso form. See e.g., U.S. Pat. Nos. 5,455,366; 5,329,033; and 5,145,819. See also, Herrmann, et al., 28 Angew. Chem. Int. Ed. Engl., 1511 (No. 11, 1989). Additionally, separation of the two isomers is inefficient because of the low yield of procatalyst, as well as the additional purification step required. Alternatively, processes have been described in which the meso form is defeated by substituting various groups onto the Cp groups. See, e.g., U.S. Pat. No. 5,349,032.
To avoid the difficulty associated with the use of polar solvents, a process which employs non-polar solvent has been used, but this process is relatively slow and does not address those specific instances in which it is intended that the metallocene procatalyst be used in stereospecific polymerizations. See e.g., U.S. Pat. No. 5,117,020.
A more desirable process would allow for the production of metallocene procatalysts at high yields but in relatively short reaction times. Additionally, such a process would allow for the production of metallocene procatalysts that could be used for the stereospecific polymerization but would not entail preparative steps in which various stereoisomers of the procatalyst are separated and would not be restricted to procatalyst that, due to substitutions, do not have a meso form as a stereoisomer.